1. Field of the Invention
The present invention relates to a method for selecting one or more transponders, in particular backscatter-based transponders, from a plurality of transponders by a base station.
2. Description of the Background Art
Selection methods, which are also called anticollision methods, are typically used in, for example, contactless identification systems or radio frequency identification (RFID) systems. A system of this nature typically has a base station or a reader and a plurality of transponders or remote sensors, which are located in a response area of the base station at the same time. If the data transmission is to take place only between one transponder or a group of transponders and the base station, a selection process must be carried out prior to the data transmission in question.
In this context, a basic distinction is made between stochastic and deterministic selection methods. A detailed description of deterministic selection methods and also stochastic selection methods can be found, for example, in the textbook by Klaus Finkenzeller, RFID-Handbuch, 3rd edition, HANSER, 2002, see especially Chapter 7.2, Vielfachzugriffsverfahren (multiple access methods), which has been published in English by John Wiley & Sons, and which is incorporated by reference herein.
In contrast to deterministic methods, stochastic methods do not presuppose a unique identification (U-ID) with a structure such as those described in the ISO 15963 standard. Assignment of such U-IDs is undertaken by bodies including a variety of manufacturer-independent organizations, for example the EAN/UCC or the IATA. However, the assignment can also be made by a manufacturer on its own. As a result, it is not always possible to ensure the uniqueness of U-IDs in open systems in which transponders from arbitrary manufacturers may be located in the response area of a base station. Stochastic methods permit selection even in these cases. Examples of such stochastic methods include the ALOHA method, the slotted ALOHA method, and the dynamic slotted ALOHA method.
The ALOHA method is a transponder-controlled, stochastic method in which the transponders transmit their data for transmission with a time offset. As a rule, the time offset is set on the basis of a random number generated in the transponder. If multiple transponders transmit an identification within the same time slot, a so-called collision occurs. This generally prevents the base station from being able to receive the transmitted data error-free.
In the slotted ALOHA method, the probability of collision is significantly reduced as compared to the plain ALOHA method. It is a base-station controlled, stochastic method in which the transponders are active, i.e. begin transmission of data, only at defined, synchronous points in time. To this end, the base station prescribes numbered time slots, or slots, and the transponders each generate a random number, with every transponder whose random number corresponds to the number of a time slot transmitting data or an identification to the base station in this time slot. To initiate the selection process, the base station generally transmits a command to the transponders, which indicates the start of a selection procedure. After receiving the command, the transponders store the applicable random numbers, which for example were previously generated or calculated in the transponder. When only one transponder transmits an identification within a time slot, this transponder is selected within the time slot, or can be selected by the base station by transmission of a command or an acknowledgement signal. The base station can then, for example, perform write and/or read operations on this transponder.
When multiple transponders transmit an identification within the same time slot, a collision occurs. Depending on the bit coding, the base station can detect such a collision immediately or after a delay, and can skip the corresponding time slot and attempt to process time slots in which no collision occurs, or can initiate a new selection procedure by sending an appropriate command to the transponders. Since the transponders typically generate or store new random numbers, the possibility exists that no collision will now occur.
The probability of collision depends on the number of transponders in the base station's response area and the number of time slots made available. Since the number of transponders can fluctuate tremendously, a static number of time slots can lead to problems. If the number of time slots is too small, the probability of collision increases sharply. If the number of time slots is too large, there are correspondingly many time slots in which no transponder transmits data. The time required for the selection process thus increases sharply in both cases. To achieve optimum throughput, the number of time slots in which the transponders transmit data should be selected to approximately equal the number of transponders.
The dynamic slotted ALOHA method, in which the number of available time slots can be controlled by the base station, was created in order to solve this problem. In this method, the base station can initiate a selection process with a small number of time slots, for example. If collisions frequently occur in this case, the base station can initiate a new selection process in which the number of time slots is increased, thus reducing the probability of collisions.
A variety of methods are known for producing a random number for the stochastic methods. Thus, for example, the time period between a reset of the transponder and the point in time when a first symbol is received can be used as a basis for calculating the random number. Other methods combine numbers from two different areas of memory in order to determine the random number, while as a further refinement, a received data item can additionally be included in the calculation.
Other methods use a linear feedback shift register for random number generation; the shift register can be operated with a clock source, which has a certain amount of dispersion between different transponders, for example. As a result of their individual clock sources, after a certain operating time, the shift registers of different transponders then exhibit different values which can be used as random numbers.
In the slotted ALOHA method, the base station defines numbered time slots and a transponder whose random number corresponds to the number of a time slot sends data or identification to the base station during this time slot.
For this purpose, a transponder customarily has what is known as a slot counter and a binary comparator in addition to the random number generator. After the initiation of the selection process by the base station, the slot counter is decremented or incremented, starting from an initial value, when the base station indicates the start of a new slot or time slot by transmitting a corresponding command. The binary comparator compares the random number present in the random number generator with the current slot number of the slot counter, and if the random number and slot number match, the relevant transponder transmits its identification to the base station. Since the random number generator and the slot counter are designed as separate units, such an implementation requires a relatively large chip area.